The UK banks, as far as I know, are unable to take Irish property as security for a UK mortgage - this even holds true for Ulster Bank who only operate in Northern Ireland and the Republic of Ireland, i.e. you cannot get a sterling mortgage in Northern Ireland for a property across the border.
Even if you could, would you really want to?
Last August, every £1 was worth €1.14. Assuming you had a €228,000 mortgage, you would have needed a £200,000 UK mortgage to remortgage to the UK.
If you decided to move back to Ireland this week and needed to switch the mortgage back, the rate touched €1.26. Therefore, you'd need a €252,000 mortgage to clear your UK mortgage.
The above ignores the currency exchange fees (which would leave you even more worse-off) and the money you'd have paid of the mortgage. It's just to show you that the mortgage interest rate itself shouldn't be your only consideration - not that you need to consider anything because it's not possible anyway.